1. Field of the Invention
The invention relates to an improved urea synthesis process comprising separating unreacted ammonia and carbon dioxide under a pressure nearly equal to the urea synthesis pressure by means of carbon dioxide stripping, wherein the synthesis column and the like can be placed on the ground.
2. Description of the Related Art
It is well-known to produce urea by processes which comprise reacting ammonia with carbon dioxide in a urea synthesis zone under a pressure and temperature of urea synthesis, separating unreacted ammonium carbamate as a mixed gas of ammonia and carbon dioxide from the resultant urea synthesis solution, absorbing the mixed gas by an absorption medium to recycle the resulting absorbate to the urea synthesis zone, and providing a urea solution from the urea synthesis solution from which the ammonium carbamate has been separated. A variety of proposals have been made regarding the urea synthesis process of this type.
Though depending on the process it employs and its production scale, the urea plant comprises a synthesis column, a carbamate condenser and a stripper, of which the synthesis column and the carbamate condenser have sometimes been placed at a height of 20-30 m from the ground level on their bottom basis. Where these equipments are placed at a height of 20-30 m from the ground level, structures supporting these equipments only by structural members such as steel frames have never been employed and concrete and the like functioning as the foundation have been constructed upto this height or a combination thereof with steel-framed structural members has been employed. In view of this background, there have been developed urea synthesis processes in which the urea synthesis column and the like are placed on the ground, by reason that it takes much time and labor to install and maintain these equipments at the height, or by the like reasons.
As an example of the processes suggesting the ground placement, an improved urea synthesis process disclosed in Japanese Patent Laid-Open Publication No. 149930/1978 is mentioned. According to this process, in an equal pressure double-flow recycle process for synthesizing urea with intermediate ammonium carbamate, which comprises reacting ammonia with carbon dioxide at a high ammonia to carbon dioxide molar ratio (hereinafter simply referred to as N/C), heat-treating the synthesis product in the presence of a stripping agent under substantially the same pressure as that of the synthesis, and recycling the remaining materials and the excess material separated from the synthesis product in two separate streams under the same pressure,
(1) the urea synthesis product is treated in two continuous steps under the same pressure as that of the urea synthesis, and in the first step, the above-mentioned product is heated to decompose substantially all of the remaining ammonium carbamate and the decomposition product is discharged together with part of the excess ammonia, while in the second step, the remaining part of the excess ammonia is discharged by the addition of supplementary heat and the introduction of a carbon dioxide stream, and PA1 (2) the gaseous phase stripped off in the first step is recycled directly to the synthesis, and the gaseous phase stripped off in the second step is subjected to condensation and a residual gas purge treatment and then recycled to the synthesis in a liquid state.
The synthesis pressure used therein is 100-250 kg/cm.sup.2, preferably 180-225 kg/cm.sup.2, and the synthesis temperature is 170-205.degree. C., preferably 180-200.degree. C.
The synthesis part comprises two separate synthesis autoclaves continuously arranged in two steps or it is comparted into two steps by completely dividing a synthesis autoclave into the upper and lower sections by means of a partition plate provided in the lower part of the autoclave. As synthesis conditions in the synthesis autoclave located above (hereinafter referred to as the first zone) and the synthesis autoclave located below (hereinafter referred to as the second zone), the N/C in the second zone should always be larger than the NIC in the first zone. The N/C is 4-7 in the first zone, while it is 6-8 in the second zone. Separately, strippers are provided in a number of two, and the excess amount of ammonia dissolved in the liquid phase is recovered in the first step of the two stripping steps and introduced into the first zone, while the gaseous phase stripped off in the second step of the stripping steps is subjected to condensation and a residual gas purge treatment, and thereafter the condensate is recycled to the first zone.
As an attendant effect of this process, it is described in the upper left column on page 6 in the aforementioned publication that by using a reactor having both the upper and lower sections, it is possible to avoid the installation of a gigantic and unmanageable scaffold that is usually necessary for installing a reactor at a height. This suggests that the autoclave is of a self-supporting type.
Among other examples of the process suggesting the ground placement is EP-0329215A1. This patent publication discloses a process in which a carbamate condenser is installed at a high ground and a part of the stripper gas is introduced into the synthesis section by means of an ejector, suggesting that the synthesis column, stripper and the like can be installed directly on the ground.
Although the above-mentioned two conventional techniques suggest the installation of the synthesis column, stripper and the like directly on the ground, they need to place the carbamate condenser at a height and sometimes to use an ejector.
Then, an illustration is made on a urea production process disclosed in Japanese Patent Laid-Open Publication No. 209555/1985. Unlike the above-mentioned two conventional techniques, this process permits the achievement of a high urea synthesis rate and the recovery of the heat of condensation of a gaseous mixture from the stripping treatment by a small heat-exchanging area. In this process, in the production of urea comprising forming a urea synthesis solution comprising ammonium carbamate and free ammonia (excess ammonia) from carbon dioxide and ammonia in excess in a synthesis zone under a pressure of 125-350 bar, decomposing at least a part of the ammonium carbamate present in the urea synthesis solution by the addition of heat and a counter-current contact with a stripping gas in a stripping zone under a pressure not higher than the pressure in the synthesis zone, removing the decomposition product of ammonium carbamate from the stripping zone as a gas mixture together with a part of the excess ammonia and the stripping gas, condensing at least a part of the resultant gas mixture in a condensation zone, and treating the urea synthesis solution having undergone the stripping to obtain a urea solution or solid urea, at least 30% of the equilibrium amount of urea attainable under the reaction conditions is formed even in the condensation zone, and the mixture containing ammonium carbamate and urea is supplied to the synthesis zone. The condensation in the condensation zone is carried out under the synthesis pressure or the pressure of the stripping treatment lower than the synthesis pressure. Further, the condensation is effected in a submerged condenser, particularly in the shellside of a shell and tube heat exchanger arranged horizontally. The condensation can also be practiced in a condensation zone integrated with the reaction zone.
In the upper left column on page 4 of the publication, however, it is mentioned that the dip-type condenser may be arranged horizontally or vertically and it is particularly preferable to carry out the condensation in a vertical submerged condenser. It is also described that where a horizontal condensation zone is applied, a synthesis zone and the condensation zone can be accommodated within an equipment so that a compact structure is obtained. However, it only suggests an example of the application of a vertical condensation zone, that is, the accommodation of the synthesis zone and the condensation zone within an equipment.
Further, there is also a description that where a horizontally arranged submerged condenser is used, it can be disposed directly on the working floor, thus lowering the height of the equipment, reducing the cost of the equipment, and facilitating its assembly and dismantlement. This description however does not teach the ground placement of the equipments.